BUGFIX: MediaBrowser's upload action shows empty flash message if file is too big

[mhsdesign]

expected (like in the standalone media browser)

actual (media browser insider neos ui):

interestingly the error works in the neos ui when one uses the media browsers drag and drop

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The existence of this bug was uncovered via #5528 because previously no validation error was shown and on upload the server just died:

Could not convert target type "Neos\Media\Domain\Model\Asset": Missing constructor argument "resource" for object of type "Neos\Media\Domain\Model\Asset".

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Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions